Vibration measuring apparatus



SEARCH ROOM SUBSTITUTE FOR MlsszNG XR on. 2o, 1942.

c. s. DRAFT-:R

VIBRATION HEASURING APPARATUS original Filed nay 18, 1937 W .l w.. O d 2 9 2 i .5. A Y f INVENTOR CHARLES S. DRAPER HIS TTORNLA/ML"l UNITED Patented Oct. 20, 1942 VIBRATION MEASURING APPARATUS Charles S. Draper, Newton, Mass., assignor to Research Corporation, New York, N. Y., a corporation of New York STATES PATENT OFFICE 7 Claims. (Cl. 17h-209) This application is a division of U. S. patent application Serial No. 143,269, led May 18, 1937, now U. S. Patent No. 2,254,172, dated August 26, 1941, for Vibration measuring apparatus. The invention disclosed and claimed herein relates to improvements in apparatus for the measurement of torsional or angular vibration and more particularly to apparatus adapted to furnish an electrical measure of such vibration.

In my copending application, joint with George P. Bentley, now U. S, Patent No. 2,251,436, dated August 5, 1941, there is disclosed novel vibration measuring and recording apparatus adapted for measuring linear vibration of a body. The said apparatus employs an electromagnetic generator having a coil and relatively movable magnet coupled by weak resilient means to provide an instrument having a low natural frequency of vibration so that relative motion of the coil and magnet generates a voltage in the coil proportional to the velocity of the vibratory motion.

The principal object of the present invention is to provide apparatus of similar character for measuring torsional or angular vibration, such as that of a rotating shaft, by converting a portion of the vibrational energy into electrical energy, which may be conveniently used for indicating or recording purposes.

Another object is the provision of apparatus of the above character which does not affect the vibration characteristics of the member investigated and which permits the measurement and recording of torsional vibration at one or more remote points by a central unit or units.

Still another object of the present invention is to provide novel vibration measuring apparatus, including an electromagnetic generator supplying an output voltage proportional to the velocity of the vibration being measured, which voltage may be applied to an integrating circuit for securing a measure of the displacement or amplitude of the vibration and thereby an indication which is independent of frequency.

A further object is to provide apparatus of the above type which supplies an accurate measure of torsional vibrations of frequencies above a known lower limit.

Other objects and advantages of this invention will become apparent as the description proceeds.

In the drawing,

Fig. 1 is a perspective schematic View of a magnetic generator adapted for use as a torsional vibration pick-up.

Fig. 2 is a sectional perspective view of a complete torsional vibration pick-up unit.

Referring now to Fig. 1, there is shown schematically a torsional vibration pick-up unit having an armature I of high permeability material such as iron or a ferrous alloy, the said armature being adapted for rigid attachment to a shaft 2, such as the crank shaft of an internal combustion engine whose torsional vibrations are to be measured. A coil of fine wire 3 is wound in a suitable centrally positioned recess in armature I and electrically insulated from the armature. Connections to coil 2 are brought out by way of slip rings 4. A magnet assembly comprising a pair of horse shoe permanent magnets 5 and a pair of pole pieces 'I is mounted with the air gap between said pole pieces, and in which said armature is positioned. coaxial with shaft 2 and said assembly is restrained from rotation relative to the shaft by Weak springs (not shown in Fig. 1). Said springs serve to couple the magnet 5 resiliently to the shaft 2 and position the magnet structure so that the arcuate inner surfaces of pole pieces 1 are normally symmetrically disposed with respect to armature I. Oppositely positioned pole pieces l are of opposite polarity and due to the described symmetrical arrangement the portions of armature I on opposite sides of coil 3 normally carry equal amounts of flux. The air gap between armature I and the pole pieces 'I is made very small so that two low reluctance flux paths are thus provided for the flux on opposite sides of coil 3. This air gap has been greatly enlarged in the picture to facilitate the illustration.

Due to the presence of the aforementioned weak coupling springs the structure comprising magnets 5 and pole pieces 'I rotates with a substantially constant angular velocity which is the average velocity of shaft 2, any tendency of the magnet structure to vibrate freely at its own natural frequency or at frequencies adjacent thereto being substantially prevented by damping means later to be described. This natural frequency of vibration is determined by the moment of inertia of the magnet structure, together with parts rigidly connected thereto, and the elasticity of the coupling springs. By a suitable choice of constants the torsional vibration of shaft 2, when it occurs at a frequency higher than approximately two or three times the natural frequency of the moving system, produces relative motion between armature I and magnet 5 substantially equal to the magnitude of the vibration, resulting in a proportional transfer of flux from one side of armature I across coil 3 to the other side. Such flux transfer relative to coil 3 induces a voltage in the coil proportional to the rate at which flux crosses the coil, and this voltage is therefore proportional to the velocity of torsional vibration of the shaft.

The voltage induced in coil 3 is applied to an external circuit such as torsional vibration velocity indicator 4, which may be of the voltmeter type, through slip rings 4. The magnet structure is one capable of being highly magnetized and owing to the small air gap employed a high iiux density is obtained through armature I and pole pieces 1. The relative movement of armature and magnet poles therefore induces relatively high potentials in coil 3 so that this electromagnetic pick-up unit is highly sensitive and efficient l in use, a small unit providing a relatively large voltage output.

Should it be desired to indicate the magnitude of the torsional vibrational displacement of shaft 2 rather than the velocity of the vibration, the output voltage obtained at slip rings 4 may be applied to an integrating circuit and by the process of integration a voltage proportional to vibrational amplitude may be derived from the velocity output. Integrating circuits suitable for connection to the pick-up of the present invention are known in the art and are shown and described in my prior joint application, now U. S. Patent No. 2,251,436, previously referred to. Likewise, should it be desired to indicate the acceleration of the torsional vibration of the shaft, the output voltage at slip rings 4 may be applied to a differentiating circuit such as is also shown and described in my previously mentioned joint application and various types of which are known in the art. By differentiation a voltage proportional to vibrational acceleration is thus derived.

In Fig. 2, there is shown a practical construction of torsional pick-up incorporating the principles of Fig. 1 and parts of Fig. 2 corresponding to Fig. 1 are similarly numbered. In this figure, the seismic element, which is that part of the pick-up rotating at a constant angular velocity, consists of a cylindrical member 6 which may be of brass and which rigidly supports permanent magnets 5 and pole pieces 'I, the magnets being poled and positioned to induce poles of opposite sign in the two pole pieces. The pick-up unit is attached to shaft 2 by means of a anged collar 8 and a shaft 9 preferably of bronze is secured to collar 8 for coaxial rotation with shaft 2. Shaft 9 is journalled in ball bearing I0 Which is housed in cylindrical member 6 of the seismic element. Cylinder 6 is coupled to flange II of collar 8 through weak springs I2 having their ends connecte'd to fiange II and cylinder 6, respectively.

The seismic element comprising cylinder 6 together with the magnet assembly mounted on it and springs I2 constitute an oscillatory system Whose natural frequency of vibration as determined by the moment of inertia and elasticity of its moving elements is materially lower than the frequency of any vibration to be measured. As noted above, the relative displacement of parts in such a system is proportional to vibrational amplitude, the electrical output of the generator actuated thereby being proportional to vibrational velocity.

Relative motion between the seismic element and flange II is limited to a small angle by a pin I3 carried by iiange I I and projecting into a slot in cylinder 6 to avoid swings of excessive amplitude due to resonance eliects or shock. A spring pressed plug I4 of suitable friction material is carried by cylinder 6 and engages flange II to furnish the desired damping of the oscillatory system. It will be apparent that a change of damping constant may be obtained by changing the strength of the spring engaging plug I4. Armature I, which is mounted on shaft 9, carries coil 3, connected electrically to slip rings 4 and brushes I5 and thence to leads I6. A housing II which may be of aluminum to reduce its weight supports brush holders I8 and is mounted on shaft 9 by means of ball bearings I9. Housing Il is stationary and therefore does not constitute a part of the seismic element. The brush assembly is held in place by a split clamp nut 20 and an insulating cap 2I overlies the end of the structure and holds leads I6 in place.

As many changes could be made in the above construction and many apparently widely different embodiments of this invention could be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawing shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. A means for converting energy of vibratory angular motion about a given axis into electric energy, comprising, in combination, a shaft rigidly connected to the vibrating structure, a balanced mass mounted on bearings coaxially with said shaft and restrained to rotate with the mean angular velocity of said shaft by springs disposed between said mass and said shaft, magnet means secured to said mass having magnet poles symmetrically disposed on either side of the axis of said shaft and mass, an armature of material having a magnetic permeability high comparedv to that of air secured to said shaft, said armature having a recess, a coil mounted on said armature in said recess and electrically insulated from tiie armature, said armature andv coil beingso disposed between poles of said magne that relative motion between said mass and said armature causes a transfer of flux from a portion of said armature on one side of said recess to a portion on the other side, thereby resulting in a change of fiux through said coil and generating an induced voltage substantially proportional to the velocity of relative motion between said mass and said armature, said voltage being therefore substantially proportional to the vibratory angular motion of this shalt above a certain frequency.

2. Apparatus for measuring torsional vibration comprising a rotatable armature, a coil mounted on said armature, a coaxially mounted field magnet rotatable relative to said armature, and resilient means coupling said armature and field magnet, said members being so constructed and arranged that relative torsional vibration of said armature and magnet causes a change in the distribution of iiux relative to said coil and thereby generates a voltage in said coil proportional to said vibration.

3. A device for measuring the angular vibration of a rotating body about the axis of rotation thereof comprising an armature assembly adapted to be mounted on and directly rotated by said body about said axis, said assembly including a. permeable core and a relatively thin coil mounted thereupon substantially in a plane through said axis, a field magnet mounted for rotation about said axis and having pole pieces of opposite polarity arranged to create ux in said armature, a resilient coupling between said armature and magnet, said coupling normally centralizing said coil relative to said pole pieces, whereby angular vibration of said body causes a voltage proportional to the velocity of said vibration to be generated in said coil.

4. A device for measuring the angular vibration of a rotating body about the axis of rotation thereof comprising an armature assembly adapted to be mounted on and directly rotated by said body about said axis, said assembly including a permeable core and a relatively thin coil mounted thereupon substantially in a plane through said axis, a eld magnet assembly having a relatively high moment of inertia mounted for rotation about said axis, a weak resilient coupling between said armature and magnet, said coupling normally centralizing said coil relative to said pole pieces, whereby angular vibration of said body causes relative angular displacement between said armature and magnet proportional to the amplitude of said vibration and thereby generates a voltage in said coil proportional to the velocity of said vibration.

5. In apparatus for measuring the motional characteristics of a rotating body, a magnet member having a ilux-permeated air gap, an armature member centrally located in said gap, means for rigidly mounting one of said two members on said body for rotation therewith, means mounting the other of said members on said one member for rotation relative thereto about the rotational axis thereof, a weak resilient coupling between said two members resisting angular displacement therebetween and a coil mounted on said armature member for generating an E. M. F. responsive to change of relative angular position of said two members.

6. In apparatus for measuring torsional vibration, a magnet having a flux-permeated air gap, an armature centrally located in said gap, means mounting said magnet and armature for relative rotation about an axis perpendicular to the direction of the magnetic eld in said gap, a coil on said armature positioned with its axis perpendicular to said rotational axis and normally perpendicular to said eld, a resilient coupling re sisting relative displacement of said magnet and armature about said rotational axis, and means for rigidly mounting one of said relatively displaceable members on a rotating body to be driven thereby.

7. In apparatus for measuring the motional characteristics of a rotating body, a magnet member having a flux-permeated air gap, an armature member centrally located in said gap, means for rigidly mounting one of sai-d two members on said body for rotation therewith, means mounting the other of said members on said one member for rotation relative thereto about the rotational axis thereof, a weak resilient coupling between said two members resisting angular displacement therebetween, a coil mounted on said armature member for generating an E. M. F. responsive to change of relative angular position of said two members, and means for damping relative oscillation of said two members.

CHARLES S. DRAPER. 

